Minnesota Soybean Symposium – Pesticide Resistance

Seth NaeveThere were some great presentations and two-way dialogue between university researchers and attendees at the 2013 Soybean Symposium sponsored by the U of M and the MN Soybean Research & Promotion Council. This event has been held at the U of M Landscape Arboretum since it’s inception.

This year’s sessions focused on resistance management of soybean insects, diseases and weeds as well as corn root worm beetles. Attendees were crop consultants, farmers, agronomists as well as a number of ag media representatives. The seed company reps may have been uncomfortable with some of the topics discussed because, if the farmers followed the recommendations, especially on soybean seed treatments, it would go against what they’re pushing on the farmers.

Aaron Hager - U of IL


Farmers are facing many resistance challenges now and on the horizon. Today’s era of post emerge only Roundup Ready crops has made a once useful chemical virtually useless against certain weeds. Most farmers today recognize this and know that changes must be made. The bigger challenge is informing farmers, seed dealers, coops, crop advisers and any other decision makers in the ag industry of what cannot yet be seen.  Ian MacRae insists that “resistance management needs to be used before resistance is seen” He defines low resistance as needing a 3X rate to kill and moderate resistance as needing a 10X rate.

Overuse of “insurance” applications of chemicals, insecticides and fungicides is very prolific in today’s high commodity priced environment. If you can spend an extra $15 on your soybeans with a the expectation of a break even or 150% return on your investment, you may take that chance because it’s $15 beans. One half bushel/acre adds up.


Bob Koch, U of M entomologist, warned of the overuse of neonictonoids such as Cruiser who’s active ingredient is thiamethoxam. While having long lasting effectiveness on insects, and possibly yields, there are many concerns about overuse of this product. These long lasting insecticides give a much higher probability of rapid resistance especially as they become common to the point of being used on up to 80% of soybean acres in the U.S. Some studies have even shown that insects are attracted to plants that have had the Cruiser seed treatment. There is excellent information available on insecticide groups at the IRAC web site. Never use a neonictonoid post emerge if it’s already been used in the field as a seed treatment!

Insecticide Resistance

By repeatedly treating soybean aphids with Lorsban for the “quick kill” benefit, we have selected for chlorpyrifos resistant spider mites. While soybeans are rarely sprayed for spider mites, we risk losing the best weapon against them if we continue to use chlorpyrifos on soybean aphids. Peak aphid populations are decreasing every year. Allowing natural enemies to work on the aphids is especially important as we face insect resistance issues. Aphids are not only attacked by predator insects but fungal infections can reduce their populations as well. This is important to remember as companies tout seed treatments (Cruiser) as an aphid control mechanism and also when being sold the need for annual application of fungicides. Will these remove natural methods of aphid control? Remember, foliar control of aphids is the most cost effective, if any control is necessary. Never use lower than labeled rates of any pesticide. Resistance will only build faster.

Something simple to remember about resistance management: when exposure is continuous, on all the time (transgenics), there is high pressure to select for resistance. Shorter residual is best for resistance management.

Seed treatments in field corn are necessary due to the fact that yields are directly related to emergence and stand. This is not the case in soybeans. Think about your reasons for treating soybeans and how that relates to future needs of a pesticide you’re using now as “insurance”.

Herbicide Resistance

You can’t miss the articles this winter in the farm media about herbicide resistant waterhemp. Please don’t take this lightly. What is your plan for escapes? More spray? What if that doesn’t work? Get out there or hire someone to get out there and eradicate them by hand! There are no new chemistries coming in the foreseeable future. Use a pre-emerge as your main control with post as a backup just like we did before Roundup. Management of field margins and ditch banks is highly important as well.

Pathogen Resistance

Dean Malvick of the U of M stated “regular use of fungicides increases the change of resistance“. There is a high risk when using single sites of action fungicides such as strobilurins (Headline, Quadris and Stratego). Risk is reduced by using multi-site mixes. Chromosome

Thank you to all involved for putting on this beneficial meeting. If you would like to comment, please feel free do to so.

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